1. Field of the Invention
The present invention relates generally to processes for treating industrial exhaust gases to remove sulfur oxides contained therein and, more specifically, to the removal of SO3 from flue gases produced by combustion of carbonaceous fossil and other sulfur-bearing fuels in such industrial processes.
2. Description of the Prior Art
The burning of fossil fuels and other fuels that contain sulfur, including pet coke, will result in the formation of sulfur oxides, most commonly know as SOx. The predominate species of the SOx is SO2, with minor amounts of SO3.
A variety of techniques have been developed over the years for removing SOx from facilities that burn fossil fuel such as coal burning electric generating stations as well as secondary industries operating coal burning or high sulfur oil-burning boilers. When there are high levels of sulfur that must be removed, the use of wet limestone or wet lime scrubbing are common and cost effective methods. This sulfur removal technology is based on removing the SOx from the flue gas at the end of the process. Whether the sulfur is present as either SO2 or SOx is generally not of material importance, since it is the amount of total sulfur removed that is critical to the sulfur removal process.
When lower levels of sulfur are present in the fuel, typically less than 2%, then injection of dry limestone or hydrated lime into the boiler is occasionally practiced. This process tends to be less efficient and thus requires a much higher dosage of calcium reagent to sulfur removed, i.e. Ca/S>2:1 or higher.
Until recently there was little or no concern as to whether the sulfur removed, or the sulfur which remained in the flue gas, was either SO2 or SO3, because most of the sulfur was believe to be SO2.
However, recently there has been a much greater concern regarding the presence of SO3 in the flue gas, even though it is a very small amount compared to the total SOx that is emitted. With the introduction of SCR (selective catalysis reduction) in coal fired power plants to control NOx, the elimination of SO3 has become a critical issue. This is partly due to the fact that an unwanted side reaction of the SCR technology to reduce NOx emissions is the catalytic reaction to form SO3. These higher concentrations of SO3 are surprisingly not being removed by traditional wet limestone or wet lime scrubbing systems, even if higher Ca/S ratios are used. This is an unexpected result considering the fact that SO3 is more reactive than SO2, and thus should be easier to remove as calcium sulfate.
The higher concentrations of SO3 accelerate corrosion of the air-heater, precipitator and dry gas duct components of the power plant. These pollutants are passing through all conventional sulfur removal systems and are causing high opacity plumes that contain fine droplets of sulfuric acid, H2SO4. This unexpected phenomena is causing major problems at coal fired power plants that are installing SCR systems.
A need exists, therefore, for a method for effectively removing SO3 from exhaust and stack gases of fossil fired power plants.
A need also exists for a method for economically and efficiently retrofitting existing power plants, especially those that burn coal, which provides pollution reduction of SO3 similar to the results achieved using wet scrubbing for SO2 reduction and electrostatic precipitation for particulate removal.